Optical head devices are well known to the art, which record and reproduce information by applying a light beam to a recording medium such as an optical disk. In a commonly used optical head device, a light beam emitted from a semiconductor laser as a light source passes through a diffracting element, and forms a parallel beam of light through a collimating lens. An intensity distribution of the parallel beam of light shows an elliptic shape in its cross section orthogonal to the optical axis, according to the intensity distribution of the light beam emitted from the semiconductor laser. Here, such an intensity distribution of light related to a cross section orthogonal to the optical axis, is hereinafter referred to simply as a cross-sectional intensity distribution. The elliptic cross-sectional intensity distribution is shaped into a substantially circular shape by a shaping prism in order to improve the efficiency of light utilization in the optical head device. The light beam having its cross-sectional intensity distribution shaped into the substantially circular shape, is converged on the recording medium by an object lens.
The light beam reflected from the recording medium (hereinafter referred to simply as a reflected beam) follows a light path in the reverse order to the above-mentioned, and has its cross-sectional intensity distribution restored to be an elliptic shape by the shaping prism. Thereafter, the reflected beam is directed to a diffracting element via the collimating lens, and diffracted by the diffracting element so as to be converged on a photodetector.
Normally, each of the diffracting element and the photodetector is divided into a plurality of regions by a plurality of division lines so as to obtain a focus error signal or a tracking error signal by adopting the knife-edge method or push-pull method. For example, in accordance with the knife-edge method, a focus error signal can be obtained by finding a difference of detecting signals released from two detecting regions adjacent to each other in the photodetector.
FIG. 16 shows one example of the FES (Focus Error Signal) curve showing the relationship between the intensity of the focus error signal thus obtained and the amount of the displacement of the optical disk based on the focal point of the object lens taken as a reference. In FIG. 16, a reference point A represents a case without a focus error. The reflected beam hits the diffracting element, and a converging point of the resulting diffracted beam directed to the photodetector varies in its position in front of or behind the photodetector in response to fluctuations of the recording medium from the position causing no focus error. As a result, since a shape of a light spot formed on the photodetector by the reflected beam is reversed, the value of the focus error signal is reversed from positive to negative in accordance with the amount of the displacement of the recording medium.
In this case, as shown in FIG. 16, when the recording medium moves away from the focal point of the object lens to a certain extent, an undesired zero-cross point B appears on the FES curve at a position other than the reference point A. Especially in an optical head device using a shaping prism in its optical system, an undesired zero-cross point may appear within a dynamic range of focusing control, depending on what arrangement is selected with respect to the optical system. This is related to the fact that the fluctuation of the recording medium causes the shaping prism to have astigmatism.
Meanwhile, the zero-cross point in the FES curve provides a drive target for the object lens in focus servo control, and if a zero-cross point B appears at a position other than the reference point A as described above, the following problems arise. When focusing control is performed so as to move the object lens from a farther position toward a closer position with respect to the optical disk, for example, in the case of occurrence of an excessive focus error due to an external cause or an initial state of the device, the object lens might be moved to focus on an incorrect target caused by the undesired zero-cross point B. As a result, normal information recording or reproduction might not be performed.
Further, besides the above problem that an undesired zero-cross point might appear in the FES curve, the optical head device has another problem that focusing control might be performed improperly due to wave-length variations of the light beam emitted by the semiconductor laser, that is, offsets might occur. In other words, when the wave-length of the light beam emitted from the semiconductor laser varies, the diffraction angle of the diffracted beam to be directed from the diffracting element to the photodetector is caused to change. Since the distance from the diffracting element to the converging point of the diffracted beam does not change, the variation of the diffraction angle results in forming the converging point of the diffracted beam in front of or behind the photodetector. As a result, even if there is no focus error, a bright portion is formed lying on either one of two adjoining detecting regions. The detecting region having the bright portion generates a detection signal. Therefore, an offset occurs since the value of the focus error signal does not become zero when it should be zero.